A group of researchers in Princeton University have managed to prove and demonstrate that disk encryption mechanism used by BitLocker of Windows Vista; FileVault of MacOS X; dm-crypt of Linux, TrueCrypt and possibly other secure encryption software, can be cracked, hacked and defeated by imaging state of physical memory (DRAM modules) which still carry and retain traces of code bits, in what hackers called cold boot attack by dumping all data in memory to disk.
Princeton University Center for Information Technology Policy website describes how the attack is possible:

Contrary to popular assumption, DRAMs used in most modern computers retain their contents for seconds to minutes after power is lost, even at operating temperatures and even if removed from a motherboard. Although DRAMs become less reliable when they are not refreshed, they are not immediately erased, and their contents persist sufficiently for malicious (or forensic) acquisition of usable full-system memory images. We show that this phenomenon limits the ability of an operating system to protect cryptographic key material from an attacker with physical access. We use cold reboots to mount attacks on popular disk encryption systems – BitLocker, FileVault, dm-crypt, and TrueCrypt – using no special devices or materials. We experimentally characterize the extent and predictability of memory remanence and report that remanence times can be increased dramatically with simple techniques. We offer new algorithms for finding cryptographic keys in memory images and for correcting errors caused by bit decay. Though we discuss several strategies for partially mitigating these risks, we know of no simple remedy that would eliminate them.

Video clip published by the team shows that it’s possible to remove a DIMM from one computer after power loss, transport and traffic the RAM module to another PC, aiding by a typical canned-air spray to lower its temperature to lengthen the time which the DIMM will keep the data, and then boot the computer unit using a specially designed microkernel, and finally dump all data on the RAM chip to physical disk. The amount of bad (decayed) data depended on both the time a DIMM spent unpowered and the temperature at which it was kept. Nonetheless, the researchers managed to successfully reconstruct 128-bit AES encryption keys within seconds, even if 10 percent of the key had already decayed out of memory.

The Princeton University team has also released the source code for some of the software utilities that is developed in the course of this research. These prototype applications are intended to illustrate the techniques described in the encryption keys cool boot attack research paper, and should not be used for malicious or hacking attempt.